A centrifuge separates and purifies a sample while the rotor holding the sample and placed in the rotation chamber is rotated at a high speed by a drive unit.
The Unexamined Japanese Patent Application KOKAI Publication Nos. 2001-104826 and 2008-23477 disclose ultracentrifuges with a rotor rotation speed of 40,000 rpm or higher. Such a centrifuge comprises a vacuum pump unit reducing the pressure within the rotation chamber to a high vacuum state and a control unit controlling the operation of the vacuum pump unit and drive unit in order to prevent rise in temperature of the rotor and sample due to frictional heat caused by windage loss between the rotor and the air in the rotation chamber.
The vacuum pump unit is constructed by series-connecting an auxiliary vacuum pump reducing the pressure from the atmospheric pressure to a high degree of vacuum such as approximately 13 Pascal and an oil diffusion pump reducing the pressure from the high degree of vacuum to an ultrahigh degree of vacuum. The oil diffusion pump includes a boiler for heating the stored oil, a heater for heating the boiler, a jet part that allows the oil molecules heated by the boiler and evaporated/gasified to pass through the center and ejects them downward in one direction from the periphery, a cooling part which cools and liquefies the high-speed oil molecules ejected from the jet part and colliding against the wall thereof and in whose lower part the surrounding gas molecules blown off by the oil molecules are compressed, an air inlet connected to the rotation chamber, an air outlet connected to the auxiliary vacuum pump, and so on.
In order to prevent rise in temperature of the rotor and sample, the control unit performs so-called vacuum standby operation in which the rotor is rotated at a predetermined low fixed rotation speed such as approximately 5,000 rpm until the rotation chamber reaches a moderate degree of vacuum such as 133 Pascal from the atmospheric pressure. Then, the control unit accelerates the rotor to a rotation speed of several tens of thousands rpm to more than a hundred-thousand rpm after the rotation chamber has reached a moderate degree of vacuum.
For centrifugal separation of a sample for which rise in temperature should be prevented as much as possible, an operator performs so-called high vacuum start operation in which the rotor is rotated only after the rotation chamber has reached a high degree of vacuum such as approximately 13 Pascal.
The centrifuge disclosed in the Unexamined Japanese Patent Application KOKAI Publication No. 2001-104826 controls the operation of the oil diffusion pump based on the temperature of the heater for evaporating/gasifying the oil in the oil diffusion pump that is detected by a temperature sensor. The centrifuge disclosed in the Unexamined Japanese Patent Application KOKAI Publication No. 2008-23477 controls the operation of the oil diffusion pump based on the degree of vacuum in the rotation chamber that is detected by a vacuum sensor.
In the above-described prior art centrifuges, it takes more than 10 minutes for the rotation chamber to reach a high degree of vacuum of approximately 13 Pascal in the high vacuum start operation. Therefore, it takes a long time before the centrifugal separation starts, leading to poor work efficiency. Furthermore, even though the pressure within the rotation chamber is reduced to a high degree of vacuum of approximately 13 Pascal, the sample temperature will be raised because of windage loss of the rotor when the centrifugal separation is performed under high centrifugal force for a prolonged time with the rotor being rotated at a rotation speed of several tens of thousands rpm to more than a hundred-thousand rpm. Consequently, in such a case, the pressure within the rotation chamber should be reduced to an ultrahigh degree of vacuum of approximately 1 Pascal.
Needless to say, a prior art centrifuge is provided with a means for maintain the inner wall surface of the rotation chamber at a proper temperature using a Peltier element or the like so as to cool the rotor rotating at a high speed. However, when the pressure within the rotation chamber is at a high vacuum state, convective air flow cannot be utilized; therefore, the rotor-cooling power is low. Then, windage loss of the rotor and frictional heat between the rotor and air should be kept low by maintaining a ultrahigh degree of vacuum around the rotor.
A powerful heater or even a cartridge heater that allows for efficient heat transfer from the heater to the oil can be used to heat the oil in the oil diffusion pump, thereby reducing the time to evaporate/gasify the oil in the oil diffusion pump and then reducing the time for the rotation chamber to reach a high degree of vacuum from the atmospheric pressure approximately to half. Furthermore, the boiler can be maintained at a high temperature so that the oil in the oil diffusion pump is vigorously evaporated/gasified, whereby the rotation chamber is maintained at an ultrahigh degree of vacuum.
However, the quantity of oil molecules evaporated/gasified and ejected from the jet part is increased as the boiler is maintained at a high temperature. In such a case, some of the gasified oil molecules are not sufficiently cooled and continuously discharged from the air outlet of the oil diffusion pump to the auxiliary vacuum pump. Then, the amount of oil stored in the oil diffusion pump is reduced and frequent oil supply maintenance service is required. Furthermore, the air outlet of the oil diffusion pump and the auxiliary vacuum pump are often connected by a rubber vacuum hose. When the heater is kept at a high temperature, the connection part between the air outlet of the oil diffusion pump (so-called elbow part) and the rubber vacuum hose is heated and an inexpensive natural rubber vacuum hose is subject to premature thermal degradation. Therefore, an expensive silicon rubber vacuum hose must be used, increasing the product cost.
The above problems can be resolved by using a powerful heater so as to allow the rotation chamber to reach an ultrahigh degree of vacuum in a short time and, once the rotation chamber has reached an ultrahigh degree of vacuum, detecting the heater temperature having a good temperature response as the boiler temperature and maintaining the boiler temperature at a proper temperature for maintaining low oil consumption of the oil diffusion pump and preventing high temperatures at the air outlet.
However, a significantly narrow range of proper oil temperatures in the oil diffusion pump realizes the above ideal state. Furthermore, for properly controlling the boiler temperature, detection errors of a temperature sensor detecting the heater temperature and other measurement errors should be taken into account. Therefore, it is advantageous that the target set temperature of the heater (the target set temperature of the oil in the oil diffusion pump) is lower than the optimum temperature.
In the above described oil diffusion pump using a powerful heater, the temperature of the oil in the oil diffusion pump will be rapidly raised by the heater and, once approached the target set temperature, stabilized at the target set temperature by controlling the temperature of the heater. Here, the temperatures of the heater and oil tend to be subject to hunting in which overshoot and undershoot are repeated with the time. The degree of vacuum in the rotation chamber is increased when the oil temperature is high (overshoot) and, conversely, is decreased when the oil temperature is low (undershoot). Therefore, the degree of vacuum of the rotation chamber also tends to be subject to hunting.
Operation of the above oil diffusion pump in a prior art centrifuge will be described hereafter with reference to FIGS. 8 and 9. FIG. 8 is an operation flowchart showing the oil diffusion pump heater control of the control unit of a prior art centrifuge by way of example. FIG. 9 is a characteristic chart showing the chronological change in the heater temperature of the oil diffusion pump and the degree of vacuum in the rotation chamber in a prior art centrifuge that was measured during the oil diffusion pump heater control in FIG. 8.
For starting the operation of the vacuum pump unit, the control unit activates the auxiliary vacuum pump to reduce the pressure within the rotation chamber and, as shown in FIG. 6, starts continuously energizing (continuously heating) the heater of the oil diffusion pump to rapidly raise the heater temperature (Step S31). Then, the control unit monitors the heater temperature corresponding to the temperature of the oil in the oil diffusion pump based on detection signals from a temperature sensor attached to the heater (Step S32) and continues to continuously energize the heater until the heater temperature reaches a target set temperature Tctl-10° C. (Step S32, NO).
Once the heater temperature reaches the target set temperature Tctl-10° C. (Step S32, YES), the control unit controls the pulse width of the electric power supplied to the heater through PID feedback control and the like so that the heater temperature becomes equal to the target set temperature Tctl (Step S33). Subsequently, the control unit continues the procedure in Step S33 until the centrifugal separation is completed, the rotor is stopped, and the energization of the heater is discontinued (Step S35, NO).
After the control unit starts continuously heating the heater, the heater temperature of the oil diffusion pump (the temperature of the oil in the oil diffusion pump) linearly rises until it reaches the target set temperature Tctl-10° C. as shown in FIG. 8. Once the heater temperature reaches the target set temperature Tctl-10° C. and the control unit moves on to the pulse width control of the electric power supplied to the heater, the heater temperature reaches the target set temperature Tctl.
However, because a powerful heater is used for heating the oil in the oil diffusion pump, the heater temperature gradually stabilizes at the target set temperature Tctl after repeated hunting between overshoot Tov1 and undershoot Tun1.
Fluctuation in the heater temperature leads to fluctuation in the oil temperature in the oil diffusion pump, which further leads to fluctuation in the quantity of oil molecules ejected from the jet part in the oil diffusion pump. Therefore, the rotation chamber reaches a target ultrahigh degree of vacuum Pmg after repeated hunting between undershoot Pun1 and overshoot Pov1. For this reason, prior art centrifuges have the problem that the time for the rotation chamber to reach a target ultrahigh degree of vacuum Pmg since the start of operation of the vacuum pump unit can not be shortened.
In this regard, continuous control for an increased target set temperature Tctl leads to the problem that the oil consumption of the oil diffusion pump is increased and the temperature at the air outlet is raised as described above. On the other hand, continuous control for a decreased target set temperature Tctl leads to the problem that the oil temperature in the oil diffusion pump is lowered and the quantity of gasified oil molecules ejected from the jet part is reduced, whereby the rotation chamber fails to reach an ultrahigh degree of vacuum in a short time.